feat(Request): Normalize wsgi.input semantics

The socket._fileobject and io.BufferedReader are sometimes used
to implement wsgi.input. However, app developers are often burned
by the fact that the read() method for these objects block
indefinitely if either no size is passed, or a size greater than
the request's content length is passed to the method.

This patch makes Falcon detect when the above native stream types
are used by a WSGI server, and wraps them with a simple Body
object that provides more forgiving read, readline, and readlines
methods than what is otherwise provided.

The end result is that app developers are shielded from this
silly inconsistency between WSGI servers.

Fixes issue #147
Merge pull request #182 from kgriffs/issues/147

feat(Request): Normalize wsgi.input semantics